US3414044A - Method of making bimetallic tubular article - Google Patents

Description

United States Patent Ohio Filed May 12, 1966, Ser. No. 549,604 9 Claims. (Cl. 164-95) ABSTRACT OF THE DISCLOSURE A method of forming a bimetallic tubular article by a centrifugal casting process. An initial pour of a chill iron alloy is introduced into a rotating mold. The chill iron alloy forms an exterior casting surface which is difiicult to machine. A flux is added over the surface of the chill iron alloy before the chill iron layer has cooled to its solidus temperature. Subsequently, the chill iron layer is cooled to a temperature below its solidus temperature and a softer iron alloy is introduced into the mold. The softer iron alloy is introduced in one or more pours and ultimately forms a readily machineable interior surface of the tubular article.

This invention relates to a method of making a bimetal- :lic cylindrical casting and to the casting resulting from the practice of the method.

The manufacture of bimetallic castings by a method which is essentially a centrifugal casting process in which the different metals are introduced successively into a spinning mold is well known in the art. It has been generally accepted that the metal first introduced into the mold must not be allowed to solidify completely at its inner diameter before the second pour is made if a sound metallurgical bond is to result between the two different alloys. See Krepps US. Patent 2,710,997 and Samuels US. Patent 2,964,251. The art, therefore, teaches that the second pour must be made while the first-poured metal is still in a plastic or mushy condition. The prior art gives examples of various compositions that can be used for the different metals, such as a steel or a chill iron outer layer and a cast iron inner layer, but has concerned itself only with relatively small articles and rolls.

Attempts to follow the prior art practice in the manufacture of large cylindrical castings of, for example, 20 to 40 inch diameter and from 200 to 400 inches long have resulted in castings that are generally defective. Such attempts have been made in a desire to obtain large roll shells having a hard white iron, or chill iron, layer backed up by one or more layers of gray iron and the roll shells have been cast primarily for use in the paper industry. The relatively. massive size of cast rolls adapted for the paper industry is such that the complications and difliculties tend to develop in connection with excessive internal strains in the castings and even cracking as a result of metallurgical differences that may be developed because of the chilling of the outer layer during the casting operation. For example, a roll shell having a longitudinal dimension of 200 inches may contract as much as 4 inches during cooling of the cast roll shells, and such substantial contraction has a tendency to develop stresses within the roll which must be controlled carefully, not only for the purpose of obtaining an initially useful casting but also for purposes of obtaining a casting which can operate under the con- 3,414,044 Patented Dec. 3, 1968 ditions of pressure, temperature, etc. required for various cast rolls in paper making machinery.

In the centrifugal casting process the mold used may be either a sand lined mold or a so-called permanent mold which is simply a metal shell having an appropriate mold wash thereon. In either event the mold is rotated to effect sufiicient centrifugal force in the molten material poured therein to overcome gravity forces and the like and to develop, in the cylindrical article, a uniform radial thickness in the poured shell throughout an annular cross section. It will be appreciated that in the making of Ibimetallic roll shells for use in large or heavy machinery, as in the case of paper making machinery, the centrifugal casting equipment will have to be substantial in size and in some respects control of the operation thereof will be complicated because of the massive size of the apparatus.

The prior .art teaches that the introduction of the metal for the internal layer should be made while the inner surface of the first-poured layer is still in a plastic condition so that a good metallurgical bond will be created between the two different metals. When the quantity of metal required to form the chill layer in roll shells of the size to which the invention pertains may exceed 20,000 lbs., we have found that there is a delay of from 30 minutes to 50 minutes required for the metal to reach the desired solidus condition. During this period substantial oxidation of the inner surface of the outer layer takes place and there is a substantial accumulation of slag, oxides and other lighter fractions on the inner surface of the outer layer. Attempts to bond the inner gray iron to this surface merely by pouring it into the mold will not be successful because of the presence of the undesirable materials on the layer of the partially solidified chill iron. The present invention not only carries solidification of the chill iron to a somewhat greater extent than is taught by the prior art but proposes the introduction of a flux prior to the time that the gray iron layer is poured after a protracted delay period as hereinafter set forth.

In the practice of the present invention it has been found possible to correlate casting conditions, including the rate of chilling, with chemical analyses of the molten iron to obtain not only a selected preferred composition of alloyed white iron in the outer chilled periphery of the shell but also a preferred composition of unalloyed gray cast iron in the inner diameter of the shell (with an intermediate mottled iron portion), whereby the components of the roll shell are firmly bonded or integrated, notwithstanding their different chemical analyses and essentially different metallurgy of the exterior and interior peripheral portions of the ultimately cast roll shell.

Where the casting has been made on a permanent mold With a conventional mold wash containing silica flour and bentonite there appears to be some tendency for porosity to appear on the exterior of the resulting shell. This porosity is throught to be attributable to the release of hydrogen from the water held by the bentonite as normally combined water of hydration, and the present in vention includes a step to eliminate such porosity by proper preparation of the permanent mold after coating,

When the white iron layer has been poured and its interior properly treated with flux to promote bonding, the gray iron can then be introduced into the mold. Care must be exercised to prevent washing away too much of the white iron by the erosive effect of the flow of the gray iron. For example, if 40,000 lbs. of gray iron must be introduced over a pouring time of only 6 minutes there is considerable turbulence set up by the incoming metal, and the erosion is particularly severe immediately under the pouring spout. According to a preferred embodiment of the invention, therefore, the usual tubular pouring spout is replaced by a pouring spout having an elongated slot through which the metal is introduced, and the elongated slot is held above an area of rammed sand that forms'a starter core. In ramming the mold initially, a shoulder is formed at the pouring end, the radial depth of which is substantially equal to the desired radial depth of the chill iron or slightly in excess thereof. Longitudinally, the starter core need be only 18 inches to 30 inches long since this is sutficient to underlie the slotted area of the spout through which the gray iron is introduced. The gray iron thus falls from the spout onto the starter core and is not permitted to fall directly on the spinning layer of white iron. As the gray iron falls upon the starter core it picks up the rotational movement f the mold very rapidly and thus fiows in over the chill iron as an already rotating mass. It has been found that by this expedient the washing away of the white iron layer by subsequently introduced metal is greatly reduced.

While the use of a starter core to spin the gray iron prior to its contact with the previously poured chill iron layer is advantageous, it may also be advantageous in long castings to pour the chill iron from one end of the mold and the gray iron from the other. One of the effects of the erosion caused by the introduction of the gray iron is to cause a variation in chill depth from one end of the finished roll to the other. For example, if the chill layer is poured to a depth of 1 /2 inches in the spinning mold, the finished roll may have a chill depth of only 1 inch at the pouring end and the full 1 /2 inches at the stopoff end due to washing away of the metal. This variation in chill depth, while it may be accepted in many instances, is undesirable in other instances because of the variation in the stresses that might be imposed on the finished roll.

Since the chill iron contains elements intended to improve its hardness, the erosion caused by the introduction of the large quantity of gray iron with the result that hard spots appear on the interior diameter of the completed roll which make it diificult for the interior diameter to be as readily machineable as would be desired and as would be the case if the gray iron in its unalloyed condition formed the interior of the roll. The present invention, therefore, in addition to the reduction of erosion resulting from the use of a starter core proposes the introduction of the gray iron in more than one pour. The reduced quantity of gray iron that is initially poured against the previously poured chill iron not only reduces the turbulent motion of the metal and washing away of the chill iron and the resulting incorporation of its hardening elements into the gray iron, but places the hardening alloying elements in an area where they can be tolerated in the finished roll. The subsequent introduction of a second layer of gray iron not only does not wash away any of the chill iron, but virtually assures a cleaner bore or interior diameter of the finished roll because the hard inclusions are not picked up as is the case where the gray iron is introduced in a single pour. The invention is described herein in conjunction with a three-stage pour, the first being the introduction of the chill iron layer with the subsequent introduction of two layers of gray iron.

The primary object of the invention is to provide an improved centrifugal casting method and resulting centrifugally cast roll shell.

Other and further objects and advantages of the invention will become apparent to those skilled in the art from the following detailed disclosure of a preferred embodiment thereof, reference being had to the accompanying drawings which show diagrammatically the steps t o be accomplished in practicing the method both on permanent mold and sandlined molds as a part of the centrifugal casting process.

In the drawings:

FIG. 1 is a side elevational view with parts in vertical section and parts broken away of a centrifugal casting mold into which metal has been poured in accordance with the present invention; and

FIG. 2 is an end elevational view, with parts broken away of the mold shownin FIG. 1 with a diagrammatic indication of mold rotating elements.

In its mechanical and method aspects, the present invention is shown in FIGURE 1 in conjunction with a cylindrical core case 10 in which a sand mole is rammed in a conventional fashion except that the face plate end of the mold is provided with an inwardly radial projection or shoulder 12 which is hereinafter referred to as a starter core. A face plate 13 is bolted to or otherwise affixed to the end of the mold at the pouring end. The far end of the mold is stopped off with a conventional stop-off plate 14. The sand lining will vary in thick ness depending on the outside diameter of the casting desired and the inside diameter of the case 10. It is p 5- sible to use a case 10 having an inside diameter of 43 inches to produce castings having an outside diameter of from 38 to 40 inches, although the preferred technique would be to match more closely the outside diameter of the casting with the inside diameter of the case leaving a sand mold core of l to 1 /2 inches. After the sand has been rammed in place it is preferably washed with a mold wash of plumbago, and this initial mold wash may be followed with one or two coats of a known Zircon mold wash to reduce the porosity of the sand.

After the mold has been prepared, it is placed in a horizontal position and set to spinning. After the mold has come up to speed the metal forming the outer layer of the casting is poured from a conventional pouring spout to the desired depth. The pouring spout is introduced through the open end of the mold in the conventional manner and the metal strikes the mold surface a substantial distance inwardly from the pouring end. This metal will form the chill layer of the casting and its chemistry is preferably as hereinafter set forth.

The chill iron layer is designated 20 in FIG. 2 and is poured to an initial depth of, for example, 1%. to 1% inches. The pouring temperature is preferably 2400 F. to 2550 F. and the pouring time is deliberately quite rapid. If the chill iron layer is to contain, for example, 21,500 pounds of metal the pouring time will be from to seconds. It is preferred to introduce the chill iron into the mold at a rate between pounds per s cond and 270 pounds per second, the limiting factors being the ability of the mold to pick up the metal in its rotation, the capacity of the pouring spout used and the desirability of smoothing out the turbulence and wave motion that will be present when the pour is made in too short a time. The metal should be picked up by the rotating mold and flow smoothly to the far end without lapping itself and without separation.

After the introduction of the chill iron layer the mold continues to rotate and the chill iron is permitted to cool somewhat to a solidus condition. The waiting time for the chill iron to become satisfactorily reduced in temperature will be from 25 minutes to 50 minutes depending on the quantity of metal poured, the chemical com position of the metal, and on the thickness of the sand mold lining which governs the heat dissipation of the metal to the outside.

Where the prior art suggests that ferrosilicon or other material should be introduced almost immediately after the pouring of the outer layer in an effort to maintain a reducing atmosphere within the mold, the present invention delays the introduction of any material tending to promote bonding between the subsequently poured metal and a previously poured metal until well after the previously poured metal has been introduced and has become reduced in temperature. The present invention includes the introduction of a flux during the waiting period and a preferred delay time for flux introduction will be from 10 minutes to 15 minutes after pouring of the chill iron where the total Waiting period is, for example 35 minutes. Thus no flux or other material is introduced into the mold until at least one-third of the waiting period between pours has expired. The mold continues to rotate after the chill iron alloy has been introduced into the mold. Thereafter, /2 lb. of soda ash flux will be introduced into the mold for each sq. ft. of exposed metal. The fiux is added over the surface of the chill iron alloy before the chill iron layer has cooled to its solidus temperature.

At the expiration of the delay period during which time the chill iron has reduced in temperature to at least below its solidus temperature, the first pour of gray iron is made. In some instances the entire charge of gray iron may be introduced into the mold at once, although it is preferred to introduce this metal in two stages and thereby reduce the erosive effect of a large quantity of hot metal striking the chill iron. To further reduce the erosive of feet of the gray iron and chill iron layers, the starter core 12 is used in conjunction with a pouring spout 21. The pouring spout 21 is closed at the end but has an elongated slot 22 in its lower side through which the metal passes. The slot 22 is preferably elongated to a dimension about one-half of the axial dimension of the starter core 12 so that the metal will be introduced rapidly but will fall almost entirely on the rapidly spinning starter core and will not impinge directly on the previously poured chill I iron layer, but will flow out over the chill iron as an already spinning sheet'flowing lengthwise of the mold. The gray iron, being introduced through the slotted pouring spout will enter the mold at a somewhat lower rate which is desirable because of the lower coefficient of friction of the chill iron layer that is already present in the mold. For example, if 10,000 lbs. of gray iron is introduced into the mold to form a first gray iron layer as much as 140 seconds may be taken to make this pour. It is thus preferred that the gray iron portion that will be in direct contact with the previously poured chill iron be introduced at a rate of from 60 to 85 pounds per second, and the pouring temperature is preferably held at about 2380 F. As is known in the art, the heat of the gray iron will remelt the inner surface of the previously poured chill iron to a point where a satisfactory metallurgical bond .will be established between the two layers without laps or cold shut defects occurring.

After the introduction of the initial gray iron layer, the present invention also includes the introduction of a sec- 0nd gray iron layeragain poured into the mold through w bonding between the two layers without defects and the relatively slow pouring rate of from 30 to 60' lbs. per second is preferred, although even slower pouring rates may be satisfactorily employed.

- After the introduction of the entire charge of gray iron the casting is SPUHILH the mold until the metal has completely solidified and reduced in temperature to the point where the casting can be removed in a normal manner, In view of the very large quantity of metal introduced into the mold this spinning time after the entire charge of metal is introduced may be as long as 4 to 8 hours.

When pouring on a permanent mold, the mold is washed in accordance with the techniques set forth in Schuh Patent No. 2,399,606. However, whereas the Schuh patent teaches drying the mold wash for a relatively short period of time the present invention includes a protracted drying of the mold wash at a relatively high temperature. For example, it has been found that if the mold Wash is dried for a period of 4 to 8 hours at 800 F. the water of hydration, or the combined water in the constituents of the mold wash will be driven off to the point where it will not interefere with the soundness of the outside of the chill iron layer. If a short period and low temperature drying cycle is used to cure the mold wash porosity in the exterior of the chill iron layer has been experienced due, it is believed, to the release of water of hydration and decomposition of the water to form hydrogen bubbles or inclusions in the skin of the chill iron. Here again, the defects are attributable to the very large quantity of metal that is introduced into the mold, Well in excess of the quantities contemplated by the normal experience of the art in the practice of the Schuh process.

Where a permanent mold is used the heat can be dissipated much more rapidly and the delay period between the introduction of the chill iron and the introduction of the gray iron can be reduced noticeably. A period of 4 to 12 minutes is usually all that is required to bring the chill iron charge down to the temperature at which the gray iron may be poured without seriously eroding the chill iron and without any substantial dilution or destruction of the chill iron. While no starter core need be used when pouring against a metal mold, it is again desirable that the gray iron pour be made relatively slowly and in two or more stages to assure that the erosion of the chill iron is reduced and that such hardening constituents of the chill iron as may be washed away and alloyed into the gray iron remain with the first gray iron layer and do not appear on the inside diameter of the roll, or the exposed surface of the last gray iron poured.

As previously noted the present invention uses soda ash as a flux, in preference to certain other conventional materials. It has been found that in making large diameter castings according to the present invention if the soda ash flux is added immediately after the chill iron is poured, a portion of the chill iron will be decarburized and the decarburized material will migrate through the spinning metal towards the outside of the mold and will solidify in the form of pearlite inclusions on the surface of the chill iron. These are somewhat softer than the remainder of the chill surface and constitute surface defects. If the addition of the soda ash is made after the chill surface has solidified sufficiently the pearlite is unable to migrate to the exterior and does not appear on the working area of the roll. However, if the addition of the soda ash flux is made too late, the fluxing action is insufficient. It has been found that in a casting containing from 10,000 to 20,000 pounds of chill iron that a delay time of 10 to 15 minutes or at least onethird of the waiting period between pours is adequate.

A typical metallurgy of the constituents as analyzed in the ladle will be as follows:

1 Less than .01.

The hardness of the chill iron constituted as above will run between 70 and on a Shore Scleroscope Scale C. The chill depth as poured should be between 1 and 1% inches in a roll having a wall thickness of 5 inches for use in large paper making machines for which the rolls are primarily intended. Attempts to greatly increase the chromium content of the chill iron have resulted in cracking of the casting during cooling and attempts to greatly increase the nickel content have likewise resulted in defective rolls.

By pouring the gray iron in two or more stages the appearance of chromium as an alloy in the gray iron at the inside diameter of the roll seems to be avoided and the machineability of this portion of the casting is retained.

What we claim is:

I l. A method of forming a bimetallic tubular article by a centrifugal casting process in which successive layers of different ferrous alloys are poured into a rotating mold to form a tubular casting having a hard exterior surface that is difiicult to machine and a readily machineable interior surface, comprising the steps of introducing into the mold in an initial pour a layer of chill iron alloy that will ultimately form the exterior casting surface, rotating the mold after the chill iron alloy has been introduced into the mold then adding a flux over the surface of the chill iron alloy before the chill iron layer has cooled to its solidus temperature, cooling the chill iron layer to a temperature at least below its solidus temperature, and introducing a layer of softer iron alloy that will ultimately form the interior surface of the tubular article.

2. The method in accordance with claim 1 with the added step of impartin a spinning motion to the softer iron in the same direction as the direction of rotation of the mold prior to the softer irons contact with the previously poured chill iron layer whereby the erosion of the chill iron layer by the softer iron is reduced.

3. The method in accordance with claim 1 in which said softer iron is introduced in at least two stages, pouring of the respective stages being separated by predetermined periods of time.

4. The method in accordance with claim 2 in which said softer iron is introduced in at least two stages, pouring of the respective stages being separated by predetermined periods of time.

5. The method in accordance with claim 1 in which the rate of pour of said softer iron is substantially less than the rate of pour of said chill iron.

6. The method in accordance with claim 3 in which the rate of pour of each stage is progressively reduced, whereby the rate of pour of each softer iron layer is less than the rate of pour of the previous softer iron layer.

7. The method in accordance with claim 1 wherein the chill iron layer is introduced into one end of the rotating mold and at least a portion of the softer iron alloy is introduced into the opposed end of the rotating mold.

8. A method of forming a bimetallic tubular article by a centrifugal casting process in which successive layers of different ferrous alloys are poured into a cylindrical rotating mold havin first and second ends to form a tubular casting having a hard exterior surface that is difficult to machine and a readily machineable interior surface, comprising the steps of applying a refractory mold wash comprising an aqueous suspension of silica flour and bentonite to the interior of the mold, heating the mold wash at an elevated temperature for a period of 4 to 6 hours, introducing into the mold in an initial pour a layer of chill iron alloy that will ultimately form the exterior casting surface, rotating the mold after the chill iron alloy has been introduced into the mold then adding a flux over the surface of the chill iron alloy before the chill iron layer has cooled to its solidus temperature, cooling said chill iron layer to a temperature at least 10 below its solidus temperature, and introducing a layer of softer iron alloy that will ultimately form the interior surface of the tubular article.

9. The method in accordance with claim 8 wherein the elevated temperature is approximately 800 F.

References Cited UNITED STATES PATENTS 1,514,129 11/1924 Clark 164-95 1,870,866 8/1932 Pike 164-288 2,255,896 9/1941 Projahn 1 64-114 X 2,710,997 6/1955 Krepps 164288 X 3,304,589 2/1967 Vologdin et al 1641 14 1,943,720 1/1934 Campbell 164-95 FOREIGN PATENTS 742,103 12/ 1955 Great Britain.

J. SPENCER OVERHOLSER, Primary Examiner.

V. K. RISING, Assistant Examiner.

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